The hormonal control of differentiation of imaginal discs of Drosophila has many features which make it a model system for the study of gene regulation. Besides the obvious possibility of applying the sophistcated technology of Drosophila genetics, biochemical studies are facilitated by the fact that apparently normal differentiation occurs in defined medium upon exposure to the steroid hormone Beta-ecdysone. Preliminary results suggest that the mechanism of action of beta-ecdysone may be basically similar to that in vertebrates and that a more detailed investigation of Drosophila is technically feasible. Discs do bind beta-ecdysone specifically and the parameters of the observed binding correlate well with morphogenetic and synthetic responses to hormone. Preliminary evidence for ecydsone receptors has been obtained. The objectives of the proposed research are to investigate in some detail the binding to nuclear and cytoplasmic components of imaginal discs, to identify the components responsible for the binding and the morphogenetic and synthetic responses. Preparations for genetic analysis will begin with the selection of appropriate temperature sensitive mutants.